It is a well-known fact that when storm water and waste water are pumped, there are greater or lesser problems of sedimentation of sand and sludge, blocking of pumps because of elongated objects and accretion of grease in pumping stations for water and waste water drains. This often causes problems such as smelly drains, including hydrogen sulphide and lumps of grease, together with sediment in the base, which has to be removed by sludge pumping. The most common type of pipe union is a single free outlet down into the draining-well, meeting the surface of the water. Depending on the water quality, various types of problems such as bad smells, bacteria spread and general fouling of the installation can occur. The ways used to minimise this problem include the installation of damping screens or down pipes which reduce the problem to a certain extent when the water runs down from the inlet pipe to the surface of the water in the draining-well. There are also prefabricated draining-wells made of glass fibre reinforced plastics, concrete, stainless steel or other materials where the drain pipe is connected at a tangent. This union is frequently dammed, i.e. at some point when the draining-well is being filled, the connection is below the normal surface of the water in the pumping station. In draining-wells made of prefabricated concrete pipes, there is great difficulty in drilling openings in a tangential direction. In the cases where the incoming flow of water falls off so much that the surface of the water in the draining-well, which is normally higher than the inlet opening, falls below the inlet opening on certain occasions, operation problems occur due to sedimentation which causes blockages. There are various types of mechanical or electrical devices in existing draining-wells, intended to stir and thus reduce the problems of blockage due to sedimentation. One large disadvantage of these device, in addition to regular maintenance, is that they require extra energy input and/or maintenance due to blockage, which leads to high operation costs.
Problems frequently occur in another type of draining-well for sewer overflow (surface water drains), which are intended to receive a mixture of storm water and waste water from a combined sewer drain pipe system to a recipient and/or treatment plant. Operation problems can occur due to (poorly sealed) leaky dams or dams which have come loose, or other devices which have come loose and can thus block the pipes. One of the most frequent causes of blockage is accretions of rags, plastic bags etc. which catch on the edge of an inlet instead of passing through the outlet pipe and on to the recipient.
One endeavours to both gain control of, and to reduce the maximum water flow that will pass on out through the outlet pipe to a treatment plant etc. This is done by choosing a relationship between bypass flow to a watercourse and to the treatment plant. For example, one can choose a high overflow edge for a small outlet dimension. If there is a low waste water flow and a small dimension outlet pipe, one obtains minimal dilution which strongly increases the risk of blockage. In order to avoid problems of waste pipe blockage, one is frequently obliged to increase the dimension of this outlet, which could be an outlet pipe or a simple hole in the sludge shield, a so-called sharp-edged outlet, which is commonly found in all conventional types of installation. By increasing the dimension of the outlet, a greater volume of water is drained via the existing sharp-edged outlet (which can be most closely compared with outlet pipe 7 in the invention), which thus becomes unnecessarily large. This means that the treatment plant will be overloaded by storm water, which should really have run over bypass edge 4. When this occurs, purification of the waste water is impaired since the treatment plant is overloaded by waste water and contaminated storm water, which should really have passed through outlet 34, directly to a watercourse. It is obvious, from the above description, that all draining-wells of the conventional type constantly represent a compromise based on an estimated average value of the waste and storm water arriving at the hopper. As soon as deviations from this estimated water flow volume occur, the hopper does not function as intended. This means that complications including several of the following can occur, such as blockage, overloading of treatment plant, release of contaminated water to watercourses, flooding of cellars and bad smells.